Gram-negative bacteria such as Myxococcus xanthus, Stigmatella aurantiaca and Escherichia coli have been found to contain a retroelement called a retron. In TIBS, 16, 18-21 (1991a), the authors report on a peculiar type of satellite DNA, named multicopy single-stranded DNA (msDNA). These molecules are characterized by a structure which comprises a single-stranded DNA branching out of an internal guanosine residue of a single-stranded RNA molecule by a unique 2',5'-phosphodiester linkage. These molecules are thus single-stranded DNA-RNA hybrids. Reverse transcriptase is required for the synthesis of these msDNAs. In Ann. Rev. Microbiol., 45, 163-186 (1991b), the authors present a comprehensive review on msDNAs. Also see msDNA in Bacteria, Lampson et al., Progress in Nucleic Acid Research and Molecular Biology, 60, 1-24.
The production of single-stranded cDNA by reverse transcriptase as a template is an obligatory step for RT-mediated transcription of retroelements. See Retroelements. See Weiner et al., Ann. Rev. Biochem., 55, 631-661 (1986) for review. This includes integration of retroviruses into mammalian genomes, production of infectious retroviruses from pro-viruses integrated into genomes, retrotransposition of retroelements, and formation of pseudo genes in eucaryotic cells.
However, single-stranded cDNAs produced in vivo by RT have never been directly detected, probably because of their instability.
While the production of msDNAs in bacteria has been a most significant development, the in vivo production of single-stranded DNAs in eucaryotic cells, e.g. , yeast or higher eucaryotic cells like plant and mammalian cells, is of even greater interest. Eucaryotes have well-known advantages over procaryotes for producing target molecules. There is an important need to produce stable single-stranded DNA in a sufficient yield for numerous practical uses in research and in industry. This invention has made an important contribution in that respect in producing single-stranded RNA-DNA structures which are detectible, stable and useful.